In the field of LED drivers for offline applications such as retrofit lamps, solutions are demanded to cope with high efficiency, high power density, long lifetime, high power factor and low cost, among other relevant features. While practically all existing solutions compromise one or the other requirement, it is essential that the proposed driver circuits properly condition the form of the mains energy into the form required by the LEDs while maintaining compliance with present and future power mains regulations. In addition, it is required that the driver circuits comply with existing power adjusting means, e.g. dimmers or the like, so that the drivers can be used universally as a retrofit driver device including the LED units.
The driver circuits should comply with all kinds of dimmers and especially the drivers should comply with phase-cut dimmers, which are preferably used to regulate the mains power with low power loss. The dimmers which are generally used to regulate the mains energy provided to a filament lamp need a low load impedance path for a timing circuit operation current to adjust the phase-cut timing. Alternatively, when this path is provided continuously, making and breaking that path for certain parts of the mains voltage cycle can also result in stable operation. The provision of this low impedance path has to be adjusted with respect to the zero crossing of the mains voltage. To achieve timely provision of this low impedance path, the zero crossing is usually detected by the driver circuit of the lamps while it is in a high impedance state. Such a zero-crossing detection is complicated and involves a high technical effort, and if a large number of LED units are connected to one dimmer circuit, the technical effort increases due to the required increase of the impedance of each individual LED unit.
WO 2009/121956 A1 discloses a lighting apparatus comprising an LED assembly and a rectifier unit to connect the LED unit to a dimmer circuit. The LED unit comprises a bleeder connected in parallel to the LED unit to provide a bleeding current. The bleeder unit is controlled by a control unit connected to the LEDs to provide the bleeding current at certain point in time of the rectifier AC voltage. This control unit is complicated and the power factor of the whole lighting apparatus is reduced due to the bleeding current.